Hi all...following the advice of Ansel Adams, Schaefer, etc I've decided to make myself a viewing card for my 5x7 to assist in composition of photographs. Schaefer recommended that I cut an opening as large as the film format I'm using....so, voila, I have a 5x7 cut out matte board.

Most of the examples that I see (Summitek 4x5 - see http://www.summitek.com/easel.html) use strings with knots that allow you to locate the frame at various distances from your eye that correspond to the field of view of your various lenses. The problem that I'm having is how to relate the distance of the card from my eye to the focal lengths of the lenses that I have since 5x7 is a different bird than 4x5. The lenses I have:

240mm Fujinon A
150mm Shcneider Super Symmar HM
450 Fujinon C

Does anybody know how I would go about creating a viewing card for 5x7 that's similar to the one shown above?